This study is based on the hypothesis that humans grafted with allogeneic bone marrow stem cells at 12-13 weeks of age will develop stable and long lasting chimerism. This hypothesis derives from the observation that T cells do not peripheralize from the human fetal thymus until 14 weeks of gestation, together with the finding that animals grafted with stem cells, before they have T cells, develop specific immunologic tolerance. Our studies will generate data on which fetal grafts of genetically engineered cells might be based.